Take this with a grain of salt for now, as similar past reports that Charlie Cox’s Matt Murdock would appear toward the end of Jessica Jones proved false, but a recent Collider Heroes showing had Heroic Hollywood’s Umberto Gonzalez claiming that Finn Jones’ Iron Fist character would show up in the Luke Cage finale (around 19:50):

Yes he is. Episode 13. That’s why it was announced, or else it would have leaked … They shot him last week.